Water Well Journal

August 2016

Water Well Journal

Issue link: http://read.dmtmag.com/i/705618

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Page 28 of 71

I came across a fellow groundwater professional working in the same area as me recently while I was on a field study. We stopped to talk. I had never met him but recognized his name from other projects. After a few moments of conversa- tion, he said, "Aren't you the guy who writes the Field Notes articles?" I was a little surprised by the question but equally flattered. It was the first time I had been asked about the article series from someone I had never met. He went on to tell me he enjoys the articles, but he thinks they are too complicated sometimes for anything he may need. Following the conversation, I considered the context and focus of these articles. In the course of contemplation, I asked a friend who is a groundwater scientist if he could explain how to work a "three-point problem." After a few minutes of attempting to do so, he finally said, "I know how to work the problem; I just can't explain it." That reinvigorated me. You see, the goal of my articles is to provide a useful tool to help everyday working groundwa- ter professionals understand the subsurface. This is regardless of your skill set and whether you work on the drilling, service, or geoscience side of the equation. The Three-Point Problem So let's work this month on understanding the "three-point problem." Estimating anything below the surface can be chal- lenging. Being able to predict the location of a specific target is near impossible—but not completely impossible. Whether you are working with an outcrop, geology map, boring logs, or groundwater—through the use of simple math- ematics, planar targets can be recognized and potentially pre- dicted with a degree of accuracy. The three-point problem, also known as the three-point method, can be used to solve for specific strike and dip of a given formation or even for the direction of movement and hydraulic gradient of groundwater. To graphically solve the problem requires the use of three known points on a common plane with corresponding eleva- tions referenced to a common datum and not aligned in a straight line. In other words, you just need to know three known locations not in a straight line with known common elevations associated with the top or bottom of a specific formation contact or planar surface. You got that? Solving the Problem To solve the three-point problem graphically, you must first create a scaled map in plan view of the planar surface that includes the three points of measurement. You can use a topographic map, a road map, computer- based surface map, or even a piece of grid paper. Once you have acquired your map, locate and mark the three points of known contact measurement. If measuring a formational contact, whether measuring an outcrop or from a boring log or geophysical log, pick the top FIELD NOTES continues on page 28 Figure 1. Three-point problem graphical exercise. Courtesy Raymond L Straub Jr., PG WWJ August 2016 27 Twitter @WaterWellJournl Solving the three-point problem By Raymond L. Straub Jr., PG FIELD NOTES FIELD NOTES

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